1. Technical Field of the Invention
This invention relates generally to wireless communication systems and, more particularly, to radio frequency (RF) transmitters used within radio transceivers of such wireless communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wire line communications between wireless and/or wire line communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet, to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), universal mobile telephone systems (UMTSs), local multi-point distribution systems (LMDSs), multi-channel-multi-point distribution systems (MMDSs), and/or variations thereof, including wireless LAN networks such as IEEE 802.11, Bluetooth, etc.
For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switched telephone network (PSTN), via the Internet, and/or via some other wide area network.
As is known by those of average skill in the art, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
Typically, in a sectored cellular network wherein each cell is divided into three or more cell sectors, each having its own amplification and transmission circuitry, beam forming antennas typically are used to create a forward link transmission pattern that fills the cell sector without overlapping in adjacent cell sectors. While one or two amplifiers could be used in a cell having more than two sectors, it is common to use one amplifier per cell sector. One problem that has been addressed by the prior art is that of amplifier failure in one of the sectors. A pair of N×N hybrid matrices are used in prior art. The first matrix will divide a signal at an input port of the first N×N hybrid matrix into N equal components, with a taper applied to each of the components. The N signals are then applied to N high power amplifiers, whereafter the amplified signals are fed to a second N×N hybrid matrix such that the original signal will only appear at one of the second N×N hybrid matrix output ports. One benefit of using the N×N hybrid matrix for this is that each signal is amplified partially by each of the amplifiers that are operational. Thus, if one amplifier were to fail, all output signals could be amplified sufficiently for transmission through all of the cell sectors (though in a degraded mode of operation). In the hybrid matrix amplifier (prior art), the hybrid matrix is fixed so that the degraded mode of operation impacts the signal-to-noise ratio. Such power sharing further has an advantage in that each forward link amplifier need not be designed to accommodate maximum power loads because additional power may be obtained from one or more other power amplifiers for maximum power requirements (across all the sectors). Thus, lower cost power amplifiers may be utilized.
FIG. 1 is a functional block diagram of a prior art cellular network cell having three cell sectors. More specifically, a cell 02 includes three cell sectors 04. Approximately in the center of cell 02 exists a base station transceiver set (BTS) 06 that includes an amplifier 08 and an antenna 10 for each cell sector 04. FIG. 1 shows the amplifiers 08 and antennas 10 well within its corresponding cell sector 04 to show the relationships therefor. It is understood, however, that the amplifiers 08 and antennas 10 for the cell sectors 04 are located approximately in the center of cell 02. The antennas 10 are so called sector antennas that radiate a pattern to fill cell sectors 04 without overlapping into an adjacent cell sector. For a system as shown in FIG. 1 in which distinct amplifiers are used but in which a hybrid matrix is not included for power sharing, each of the amplifiers 08 must be designed to satisfy maximum power level demands for the sector.
FIG. 2 is a prior art transmitter that includes a pair of analog hybrid matrices. A baseband radio 14 produces a plurality of digital waveform signals to a digital-to-analog conversion module 16 to generate a corresponding plurality of analog signals. The plurality of analog signals are then up-converted by a plurality of mixers 18 that up-convert the plurality of analog signals by multiplying the baseband signals with a local oscillation signal to create output RF signals. The output RF signals are then produced to a first hybrid matrix 20 that produces a corresponding number of transformed signals. More specifically, if the first hybrid matrix 20 receives signals sig_1, sig_2 and sig_3, it produces three transformed analog signals having components of all three signals sig_1, sig_2 and sig_3.
A power amplifier module 22 includes a plurality of power amplifiers that are coupled to receive the 1st, 2nd, and 3rd transformed analog output signals from the first hybrid matrix 20 and amplifies them. A second hybrid matrix 24 then receives the 1st, 2nd, and 3rd transformed and amplified signals and recombines them to create amplified versions of sig_1, sig_2, and sig_3 at the second hybrid matrix 24 outputs. In operation, the second hybrid matrix 24 adds the signals at the sum port and cancels out signal portions at the output ports of the second hybrid matrix 24. To effectively cancel unwanted signal components at the output ports, however, the relative component vector (phase and amplitude) and delay must be as expected. If a vector and/or delay error is introduced in or between the first hybrid matrix 20 or the second hybrid matrix 24, then perfect cancellation does not occur at the undesired ports and a resulting waveform continues to include components of other waveforms. Accordingly, it is desirable to substantially cancel the undesired include components of other waveforms thereby optimizing the signal-to-noise ratio at each output port.
While utilizing hybrid matrices are advantageous for the described reasons, including power sharing, hybrid matrices are analog devices that introduce magnitude and phase errors in the output RF signal. Accordingly, what is needed is a system that allows for power sharing to achieve the benefits of an analog hybrid matrix amplifier but that continuously compensates for the introduced magnitude and phase errors.